The present invention relates to a plasma treating apparatus for etching a silicon based substrate such as a semiconductor wafer by a plasma.
In a process for manufacturing a semiconductor wafer to be used for a semiconductor device, a thinning process is performed for mechanically polishing the back face of a circuit formation surface and thinning the thickness of a substrate to reduce the thickness of the semiconductor device. As a result of the mechanical polishing or grinding process, a stress layer including a microcrack is generated on the surface of the semiconductor wafer. In order to prevent a reduction in the strength of the semiconductor wafer from being caused by the stress layer, an etching treatment for removing the stress layer is carried out after the mechanical polishing or griding. Plasma etching has been investigated in place of a conventional wet etching treatment that uses chemicals. Plasma etching eliminates the dangers associated with the use of chemicals as well as the generation of industrial wastes.
In the plasma etching treatment intended for silicon, it is necessary to generate a plasma having a high density in order to realize a higher etching rate. Therefore, a method of jetting and supplying a gas for plasma generation having a comparatively high pressure (which will be hereinafter referred to as a “gas”) onto the surface of a semiconductor wafer is used. As such a method, conventionally, there has been known a method of using, as a gas supplying port, an upper electrode provided opposite to a lower electrode holding a semiconductor wafer and causing a discharge electrode plate to also serve as a gas introducing plate. In this case, the discharge electrode plate having a large number of fine gas supplying holes formed thereon is attached to the upper electrode, thereby uniformly supplying a gas to the surface of the semiconductor wafer.
In the case in which the discharge electrode plate is used, however, there are the following problems. In the method of jetting and supplying a gas from a gas supplying hole, the uniform distribution of the gas to be supplied is restricted and the amount of the gas to be jetted onto the surface of a semiconductor wafer is not uniform between a portion provided under the supplying hole and other portions.
For this reason, an abnormal discharge in which a plasma is convergently generated in the vicinity of the supplying hole is apt to be induced and various drawbacks are caused by the abnormal discharge. More specifically, etching is convergently performed in a portion in which the abnormal discharge is generated. Therefore, there is a drawback of decreased quality of etching, for example, a semiconductor wafer is damaged or an etching result has a variation. In addition, there is such a drawback that a discharge electrode plate provided with the gas supplying hole is worn by the plasma, depending on the material of the discharge electrode plate.
Moreover, the etched surface of a silicon based substrate does not become a clear mirror-like finished surface and a slightly opaque appearance portion is generated with a spot pattern.